There are systems that are commercialized in recent years, in which TV broadcasting or analog video signals are A/D-converted, and signals are digitally transmitted by radio so that the TV broadcasting is viewed by a monitor. In the above systems, a data transmitter functions as an access point (AP), and a monitor functions as a station (STA).
The operations of the AP and the STA in the system are briefly explained below. At first, the AP periodically transmits a beacon when power is on, and waits for an association from the STA. On the other hand, the STA detects the beacon to thereby recognize the presence of the AP, and transmits an association signal to the AP to promote the registration of the own device. Thereafter, the STA transmits a signal to request the start of transmission of desired data. The AP starts transmitting specified data based on the signal.
When requesting the AP to stop data transmission, the STA transmits a signal requesting the instruction to stop data transmission to the AP, and then transmits a disassociation signal to delete the registration of the own device.
In the system, however, even if the STA is off, the AP periodically and continuously transmits the beacon, namely the AP is in a standby state in which the AP always waits for an association from the STA. Therefore, it is difficult to achieve low power consumption.
On the other hand, Patent document 1 explained below discloses a method of reducing power consumption in an access point (AP), the reduction having been difficult to be achieved in the system. The method describes the case where a reception-level-detection comparison block is provided to compare a reception level with a predetermined reception level which is previously defined to reduce power consumption in a wireless LAN access point, and where the system is operated in either one of operating modes of a sleep mode (power-saving operation mode) and an active mode (normal mode) according to the comparison result of the reception-level-detection comparison block. The method further describes that if no data is transmitted or received over a predetermined time period during operation in the active mode, the AP switches to the sleep mode.
In Patent document 1 explained below, the AP supplies power only to the reception-level-detection comparison block even in the sleep mode. In other words, the AP does not perform a reception process such as demodulation of signals and analysis of frames in the sleep mode. Furthermore, the AP does not also perform a transmission process in the sleep mode. These features allow realization of low power consumption.    Patent document 1: Japanese Patent Application Laid-Open No. 2001-156788